Proper control of cell proliferation and differentiation is crucial during brain development, and abnormalities of neuronal production can have devastating consequences including microcephaly, mental retardation, schizophrenia, cortical malformations, and epilepsy. Signaling pathways important during embryonic development also may regulate neuronal function and cell death in neurodegenerative diseases such as Alzheimer's disease. Understanding the relationship between proliferation, differentiation, and neuronal death, and the cellular and molecular mechanisms that control these decisions has profound implications for developmental neuroscience and understanding disorders of the human nervous system. The proposed studies will explore the hypothesis that beta-catenin-mediated transcriptional activation regulates the fundamental processes of neuronal migration, neuronal differentiation, and neuronal survival. Specific Aim 1 will test the hypothesis that beta-catenin activation regulates neuronal migration. We will express stabilized beta-catenin in the developing brain broadly using beta-catenin transgenic mice and in limited numbers of cells in wild-type brains using in vivo electroporation and retroviral transduction. We will characterize patterns of neuronal migration by neuronal birth dating and examining expression of neuronal and cortical layer-specific genes using immunohistochemical techniques and in situ hybridization. Specific Aim 2 will test the hypothesis that that proliferation of neural precursors is regulated by beta-catenin activated gene transcription. Dominant negative proteins (Cadherin, TCF) and negative regulators of beta-catenin-mediated transcriptional activation (Chibby) will be expressed in neural precursors using retroviral transduction and transgenic mice, and neural proliferation and differentiation will be examined. Specific Aim 3 will test the hypothesis that persistent activation of beta-catenin signaling pathways leads to increased neuronal cell death and synaptic dysfunction that can underlie neurodegenerative disease. Gain-of-function experiments with transgenic mice expressing persistently increased levels of beta- catenin in neurons and complementary loss-of-function studies in conditional knock-out mouse models will be performed. Cell proliferation, cell cycle exit, cell death and synaptic architecture will be examined by immunohistochemical techniques, and light and electron microscopy.